FM broadcasting is currently the most important broadcast system in the world. Analogue broadcast receivers have been developed for a long time with the result of highest performance receivers. New technology offers the possibility to use new algorithms for the reception of FM broadcast signals. In particular, the increasing processing power and decreasing costs of digital processors (DSPs) offer possibilities to process analogue broadcast systems like FM and AM digitally. The digital signal processing of analogue systems offers many adavantages. The receiver size can be decreased by integration of functionality into one IC and digital broadcast systems like DRM or DAB can be integrated into the same LSI.
In FM broadcasting a multiplex signal is frequency modulated. FIG. 4 shows the spectrum of a multiplex signal. The multiplex signal consists of a sum signal and an amplitude modulated difference signal with suppressed carrier. The sum signal contains the information of the left+right audio signal and the difference signal contains the information of the left−right audio signal. To allow a demodulation of the amplitude modulated difference signal a pilot carrier is added to the multiplex signal.
In mobile FM receivers the reception situation is often bad. Current FM receivers switch from stereo reception to mono reception to gain a signal to noise ratio (SNR) of about 19 dB based on the fieldstrength and multipath detection. Such a denoising is based on the fact that the frequency demodulator output noise power spectral density is increasing squared to the frequency, i.e. that the mono signal which is equal to the stereo sum signal contains less noise than the stereo difference signal which is transmitted in a higher frequency range.
Since the switching from stereo to mono is mostly clearly audible, most FM receivers for mobile reception use a sliding stereo to mono transition. Such a sliding stereo to mono blending is disclosed in DE 44 00 865 C2 according to which the stereo channel separation might be continuously reduced dependent on information about the program type as well as a noise level within the signal and/or the fieldstrength of the frequency modulated RF-signal. Further, U.S. Pat. No. 5,253,299 and EP 0 955 732 A1 disclose to divide the stereo difference signal into several subbands and to perform the stereo to mono blending, i.e. the reduction of the stereo difference signal, independently in each subband. Such a frequency selective stereo to mono blending improves the signal to noise ratio and the channel separation of a FM receiver, especially when low RF signal amplitudes are received. However, all these systems have in common that the reception conditions are the main factor for performing the denoising stereo to mono blending which might lead to an unwanted reduction of the channel separation in many cases.